1. Technical Field of the Invention
The present invention relates to a manufacturing method for an oriented film of polyvinyl alcohol derived films used for a polarizing film etc. Furthermore, the present invention relates to a polarizing film comprising an oriented film obtainable by the manufacturing method for the oriented film concerned. A polarizing film is used as a polarizing plate etc. for a visual display, such as a liquid crystal display, an organic EL display, a PDP (plasma display panel).
2. Description of the Prior Art
Conventionally, an oriented film of polyvinyl alcohol derived films has been used as a polarizing film for a liquid crystal display etc. As a manufacturing method for the oriented film concerned, a wet type stretching method and a dry type stretching method may be mentioned. Since moisture percentage of a film gives some effects to stretching, in the wet type stretching method, a stretch irregularity is easy to be obtained in an oriented film. On the other hand in the dry type stretching method, since stretching is conducted in away that tensile stress is given, using a ratio of peripheral velocity between stretching rolls, to a film heated to be stretched at no less than a glass transition temperature, some un-uniformity based on a deformation by the tensile stress may be arisen when the film is stretched thinner, and, as a result, a stretch irregularity may easily obtained. In a polarizing film using the oriented film that has the above described stretch irregularity, problems of color irregularity and performance irregularity may possibly occur.
Japanese Patent Publication No. 2731813 etc. is propomed to a problem of a manufacturing method for an oriented film by the above described dry type stretching method. In a method given in the official report concerned, as shown in FIG. 2, before a film 1 contacts a heating roll 2, a stretch is performed in a way that a film 1 is contacted to a heating roll 2, while a tension is given to a film 1 from a direction backside, using a peripheral velocity difference between the heating roll 2 (high-speed roll) and a low-speed roll 4 configured behind of the heating roll 2 in a running direction of the film 1. According to this method, since a stretch starting position y is located on one line in the width direction, a stretch is completed in an instant and, as a result, a uniform stretch is possibly obtained.
However, in the above-described method, since a rapid stretch is given in an instant, a tension at the time of a stretching reaches a high value. A stretch needs to be performed by fully sticking a film 1 to a heating roll 2, in order to solve the problem, and then an adjustable range of stretching conditions is limited. That is, in the above described method, if the tension at the time of a stretch is set low, a floating-up of the film 1 is arisen on the heating roll 2, and conversely if it is set too high, a breakage at the time of stretching will be given. For this reason, an adjustable region for a stretching ratio is forced to be narrow.